1. Field of the Invention
This invention relates to an optical transmission device that transmits optical signals, and in particular, to an optical transmission device that can be selectively used for multiple optical fiber shaving different core diameters such as a (silica) glass fiber for long and middle distance transmission and a plastic fiber for short distance transmission.
2. Description of the Related Art
The optical transmission device transmits a light emitted from a light source with an optical fiber, and a vertical cavity surface-emitting Laser diode (hereinafter referred to as VCSEL) is employed in the light source. VCSEL has following advantages. The threshold voltage is low. The power consumption is small. A circular optical spot is easily obtainable. A two-dimensional array can easily be realized.
A large drive current is required to obtain a large optical output in the optical transmission device. However, there is a problem in that the large drive current shortens the life of VCSEL. In order to solve the above-mentioned problem, Japanese Patent Application Publication No. 2003-152284 (hereinafter referred to as Document 1) discloses a technique to prevent the life of VCSEL from becoming shorter. Multiple VCSELs are arranged on a chip to be driven simultaneously so that a quantity of light entering the optical fiber may be kept constant, while keeping the drive current for each VCSEL small. The life of VCSEL is thus prevented from becoming shorter.
Japanese Patent Application Publication No. 8-340156 (hereinafter referred to as Document 2) discloses another type of VCSEL. An upper electrode having four square cavities is provided on a square end face of a single square pillar formed on a substrate. An upper reflecting mirror is provided to cover the four square cavities. This configuration makes it possible to arrange a light-emitting spot in close proximity and make a reflection angle small.
Japanese Patent Application Publication No. 2000-299534 (hereinafter referred to as Document 3) relates to an optical transmission device that includes a two-dimensional laser array provided on a simple square so as to make lights emitted from the lasers enter a multimode optical fiber.
It is well known that there are three optical fibers to transmit the light from VCSEL. There are a single-mode glass optical fiber for long distance communication (hereinafter referred to as SMF), a multimode glass optical fiber for middle distance communication (hereinafter referred to as MMF), and a plastic optical fiber for short distance communication (hereinafter referred to as POF). Typically, the core diameter of SMF is small, for instance, 12.5 μm. SMF is excellent in transmission efficiency, but comes high in cost. The core diameter of MMF is greater than that of SMF, for instance, 50 μm or 62.5 μm. MMF is inferior to SMF in the transmission efficiency, but low in cost. The core diameter of POF is relatively great, for instance, 200 μm or 1 mm. POF is inferior to SMF or MMF in the transmission efficiency, but very low in cost.
The above-mentioned optical fibers are employed according to the purpose. For example, SMF, which has a small light loss and is used for the long distance communication, is employed in a backbone of an optical communications system. Then, SMF is changed to MMF or POF, which are used for short and middle distances, on an optical switching hub. The optical signals are provided from the optical fibers for a home networking or a LAN.
The above-mentioned conventional optical transmission devices, however, have following drawbacks. First, in the case where multiple VCSELs are arranged on the chip as described in Document 1 and are connected to the optical fiber, the multiple VCSELs have large light-emitting points or large light source. This great light source can be applied only to the optical fiber having a large core diameter such as POF. The same is true of the optical transmission devices of Documents 2 and 3. A downsized optical system can be created with the use of an optical lens; however multiple lenses are required for eliminating lens aberration, and so the cost will be increased. In addition, if a reduction ratio becomes greater than a certain value, an incidence angle becomes greater and causes the light loss. This results in a degradation of optical transmission efficiency.
Further, in the case where the backbone SMF is changed to MMF or POF on the optical switching hub as described, the respective optical transmission devices for MMF and POF need to be prepared in advance. This is because MMF and POF have different core diameters. It is not flexible in connecting the optical fiber and the optical switching hub. This causes a problem in that the cost of the optical switching hub or other communication devices that include the optical switching hub is not decreased.